Printed circuit boards (PCBs) usually have power supply circuits which provide power supply for electronic components. The power supply circuit includes a voltage regulator and a number of metal-oxide-semiconductor field-effect transistors (MOSFETs). The voltage regulator alternately turns on and turns off the MOSFETs and outputs different working voltages to the corresponding electronic components. If the power supply circuit does not have protection capability when the working voltage is over voltage or under voltage, the MOSFETs tend to be destroyed.